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Tuesday
May242011

Geophysical stamps

About a month ago I tweeted about some great 1980 East German stamps I'd seen on eBay. I impulsively bought them and they arrived a couple of weeks ago. I thought I'd write a bit about them and the science that inspired them. This week: Gravimeter.

East Germany in 1980 was the height of 'consumer socialism' under Chairman & General Secretary Eric Honecker. Part of this movement was a new appreciation for economic growth, and the role of science and technology in the progress of society. Putting the angsts and misdeeds of the Cold War to one side, perhaps these stamps reflect the hopes for modernity and prosperity.

← The 20 pfennig stamp from the set of four 1980 stamps from the German Democratic Republic (Deutsche Demokratische Republik). The illustration shows a relative gravimeter, the profile one might expect over a coal field (top), and a cross section through a coal deposit. Braunkohlenerkundung translates roughly as brown coal survey. Brown coal is lignite, a low-grade, low maturity coal.

There are two types of gravimeter: absolute and relative. Absolute gravimeters usually time the free-fall of a mass in a vacuum. The relative gravimeter is also a simple instrument. It must be level to measure the downward force, hence the adjustable legs. Inside the cylinder, a reference body called a proof mass is held by a spring and an electrostatic restoring force. If the gravitational force on the mass changes, the electrostatic force required to restore its position indicates the change in the gravitational field.

Fundamentally, all gravimeters measure acceleration due to gravity. Surprisingly, geophysicists do not generally use SI units, but the CGS system (centimetre–gram–second system). Thus the standard reporting units for gravimetry are not m/s2 but cm/s2, or gals (sometimes known as galileos, symbol Gal). In this system, the acceleration due to gravity at the earth's surface is approximately 980 Gal. Variations due to elevation and subsurface geology are measured in mGal or even µGal.

Image credit: David Monniaux, from commons.wikimedia.org, licensed under CC-BY-SA

Some uses for gravimeters:

  • Deep crustal structure (given the density of the crust)
  • Mineral exploration (for example, low gravity due to coal, as shown on the stamp)
  • Measuring peak ground acceleration due to natural or induced seismicity
  • Geodesic measurement, for example in defining the geoid and reference ellipsoid
  • Calibration and standards in metrology

Modern relative gravimeters use the same basic engineering, but of course has much better sensitivity, smaller errors, improved robustness, remote operation, and a more user-friendly digital interface. Vibrational noise suppression is also quite advanced, with physical isolation and cunning digital signal processing algorithms. The model shown here is the Autograv CG-5 from Scintrex in Concord, Ontario, Canada. It's designed for portability and ease of use.

Have you ever wielded a gravimeter? I've never met one face to face, but I love tinkering with precision instruments. I bet they pop up on eBay occasionally...

Next time I'll look at the the 25 pfennig stamp, which depicts a sonic borehole  tool.

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Reader Comments (3)

"Have you ever wielded a gravimeter?" I took Solid Earth Geophysics in the mid '80s, and one of the labs involved a portable (relative) gravimeter. I was astonished that we could locate a storm drain simply by its mass deficit! There's an area in central Oregon where I suspect a large concealed lava tube, and it's been a day dream of mine to lay my hands on a gravimeter to satisfy my curiosity.

May 24, 2011 | Unregistered CommenterLockwood

@Lockwood: Surprisingly there aren't any on eBay today -- even though there are lots of scanning electron microscopes! Most smart phones and laptops contain a tiny relative gravimeter: a micro-electro-mechanical system (MEMS) chip. This is one of the things they use to measure motion. Here's an app that measures gravity. I don't imagine they're all that sensitive, but I bet if you had a few of them all talking to each other you could eke a decent signal out of the noise. It would be a fun computer science — geophysics collaboration!

Does anyone else read gravimeter and think, 'Mmmm, gravy-meter'? No? Just me then...

May 25, 2011 | Registered CommenterMatt Hall

Spent a great summer working for BGS collecting gravity data in the Southern Uplands of Scotland. Even detected an earthquake one day!

May 27, 2011 | Unregistered Commentermatt doc

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